1. Field of the Invention
The invention pertains to a method and system for conditioning, cleaning and reconditioning or processing a heterogeneous collection of electronic components in a Fire Control Radar (FCR) high frequency, high voltage dual mode radar transmitter used in state-of-the-art military aircraft including the F-15, F-16, F-18 and B-1 bombers. More particularly the invention relates to a method and system for cleaning heat transfer surfaces of previously repaired and repairable FCR APG-68 tactical radar units and for removing embedded moisture and absorbed moisture in newly manufactured and assembled units as well as from previously repaired and repairable FCR APG-68 tactical radar units to improve their normal life or to increase their normal repaired operational life from a few hundred hours or less to an expected life of about 500 hours or greater.
In one embodiment of the invention the novel method involves cleaning the repaired units by forcing cleaning fluid through the air passages of the cold plates and heat exchangers of the transmitters to remove debris from air passages prior to extensive drying of repaired units. As described herein the step of forcing a cleaning fluid includes forcing cleaning fluid in one end with positive or negative pressure and removing the fluid at the other end with a positive or negative pressure differential. The step of forcing a cleaning solution also includes forcing an aerated cleaning solution.
In another embodiment of the invention moisture is removed from newly manufactured units by extensive drying. In all embodiments extensive drying is achieved without damaging the heterogeneous collection of electronic components in the FCR APG-68 tactical radar units by utilizing temperatures between 40 and 105 degrees Celsius for periods of time from about 2 hours to 96 hours and preferably 4 to 48 hours when employing a vacuum pressure between 0.1 Torr and 10,000 milliTorr and preferably below 100 milliTorr and then sealing such electronic components or reassembling and filling the FCR APG-68 tactical radar unit with dry gas within about 1 to 30 minutes and preferably less than 5 minutes after treatment and while the unit is still warm or above 50° C.
In a further embodiment of the invention the novel method involves removing absorbed and some adsorbed moisture from the heterogenous collection of components in new units or units which have not accumulated significant amounts of embedded moisture. These units are generally new units, units that have been recently treated in accordance with the alternative embodiment of the invention to remove embedded moisture or units that have not been repeatedly repaired and left exposed to moisture-laden environments or atmospheres. Units treated in accordance with this embodiment of the invention are treated by removing the pneumatic fill valve generally referred to as a Schrader valve and the pressure relief valve and placing the new, recently remanufactured or repaired unit in a heating and evacuation chamber at a temperature of from about 40 to 100° C. and preferably 70° C. to 85° C. for a period of about 2 to 24 hours and preferably about 3 to 4 hours at a pressure less than 750 milliTorr and preferably less than 100 milliTorr. This application of the invention removes volatiles and adsorbed and lightly absorbed moisture and reduces the total moisture content of new, recently repaired or remanufactured units that have been treated to remove embedded moisture or units that have not been exposed to moisture-laden environments or atmospheres.
In an alternative embodiment of the invention new units or units that have been treated in accordance with the alternative embodiment of the invention to remove embedded moisture or units that have not been repeatedly repaired in moisture-laden environments may be treated in a heating chamber which has a vacuum line communicating with the pneumatic fill valve port and the pressure relief port of an assembled tactical radar unit which has been placed in a heating chamber. This alternative method of the invention eliminates the need for a combined heating and evacuation chamber and allows for the heating of the heating chamber at a temperature of about 40° C. to 100° C. and preferably at about 70° C. to 85° C. for a period of about 2 hours to 24 hours as the vacuum line attached to the fill valve port and the pressure relief port is evacuated to a pressure of less than 750 milliTorr and preferably less than 100 milliTorr.
2. Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
High power radar transmitters fail periodically in service and are returned to depots for repair. At the depots the sulfur hexafluoride (SF6) is removed from the high voltage high frequency power supply or high voltage section, which is enclosed in a sealed pressure vessel of the FCR APG-68 tactical radar dual mode transmitter. The pressure vessel is then opened and the electronic components within the high voltage section are exposed to the atmosphere of the shop while the failed component(s) are being located and replaced. The system is sometimes left open for days and even weeks. After reassembly the high voltage electronic package is subjected to tests at high voltage while immersed in a bath of volatile dielectric fluid such as Fluorinert™ and subsequently is sealed Into the pressure vessel, which is then evacuated, heated and dried under vacuum and refilled with sulfur hexafluoride (SF6). After being tested the transmitter is returned to service.
It has been discovered by the inventors that the prior art evacuation heating and drying procedures removed only superficial moisture, were not effective in removing residual volatile dielectric fluids such as Fluorinert™. It has also been found that cleaning procedures to restore the heat transfer characteristics of the cold plate and heat exchanger have not been employed by the prior art. It has also been discovered by the inventors that removal of debris from and a through cleaning of the cold plate and the heat exchanger can also play a significant role in the operational life of tactical radar dual mode transmitters.
One of the other problems not recognized in the prior art is that ground testing did not simulate long period testing under actual temperature conditions encountered in flight operations. Ground testing, while adequate for demonstrating operability of the reassembled unit, did not include actual operational conditions where high ground temperatures followed by rapid low temperature flight conditions resulted in changes in vapor pressure inside the sealed unit that are caused by two types of moisture, adsorbed moisture and embedded moisture left in the unit, that reduce the life of the FCR APG-68 unit in service.
The best known prior art involves the original manufacture of the FCR APG-68 dual mode transmitters. In the original manufacture of the transmitters the partially assembled electronic assemblies (FIG. 6) were tested for corona discharge and other electrical characteristics while immersed in baths of Fluorinert™, then washed with Fluorinert™ to remove contaminants, from the electronic components as opposed to the heat exchanger components which electronic components were then dried and sealed into pressure vessels. After assembly the pressure vessels were evacuated to remove air so that they could be filled with sulfur hexafluoride (SF6). Fluorinert™ evaporates without leaving any residue, but has a boiling point such that not all of it evaporates immediately. At first Fluorinert™ residues contaminated the oil of vacuum pumps used for the subsequent evacuation and interfered with reaching a vacuum level in the milliTorr range. This problem was eliminated by adding the step of vacuum baking the electronic assemblies to remove Fluorinert™ prior to sealing the assemblies within the pressure vessels, evacuating the pressure vessels and filling them with SF6.
As manufactured relatively early in the FCR APG-68 program, the High Voltage, High Frequency Power Supply unit shown in FIG. 6 was vacuum-baked in an inverted position for about two hours, then its cold plate 50 (FIG. 5 and FIG. 6) was sealed to the aluminum high voltage pressure vessel 42 (FIG. 4) by means of an O-ring located just inside of the bolts 46 (FIG. 3). The closing and sealing was carried out while the assembly was still warm and was surrounded by an atmosphere consisting largely of nitrogen. This open vacuum-baking process unknowingly and unwittingly removed a lot of the moisture originally present in the components and absorbed during initial manufacture. Once repaired any moisture left at the time of original manufacture combined with the moisture absorbed from the atmosphere during the current repair which also added to the moisture adsorbed during previous repair operations to form harmful absorbed and embedded moisture that resulted in decreasing mean time between failures (MTBF).
In the prior art repair process Fire Control Radar FCR APG-68 units are repaired and a final process performed on repaired transmitters is to evacuate them through a Schrader valve and a Schrader valve actuator while they are being heated and then to backfill the high voltage high frequency power supply sealed within its enclosing pressure vessel with SF6. The vacuum is drawn through passages in the Schrader valve that are only about 0.060 inch in diameter and whose conductance is, therefore, very low. As a result it is believed that only a small amount of moisture and possibly only the moisture already in the air within the pressure vessel is removed at the time of evacuation and heating. The bulk of the moisture that has been absorbed from the atmosphere in the shop during the repair process remains embedded in the various electronic components, largely in organic insulating materials and builds up as embedded moisture as a consequence of repeated repairs.
Over the last twenty years the mean-time-between-failure (MTBF) of the transmitters has been falling from over 500 hours of operational life to values in the low hundreds of hours. Frequently transmitters now fail after only a few tens of hours of operation after having been serviced and ground tested. Many of the FCR APG-68 units have therefore been repaired dozens of times with each repair likely adding to the total moisture embedded in the high voltage high frequency power supply.
In addition many of the FCR APG-68 units have been repeatedly repaired and in service for thousands of hours. Some of this time has been spent on runways and in dusty locations with insects and residue from spent jet fuel. Dust-laden and residue from spent jet fuel air has been drawn through the heat transfer passages of the cold plate 50 (FIG. 3) and of the heat exchangers used to cool the Coolanol™. This dirty air and other debris has contaminated and accumulated on the heat transfer surfaces. Such contamination reduces the cooling capacity of the cold plates and heat exchangers, resulting in hot spots and an increase in the operating temperatures of the electronic components mounted to and cooled by the cold plate 50 (FIG. 6) as well as an increase in the temperature of the Coolanol™ used to cool the TWT.
The best known prior art which was employed during the original manufacturing process did not have as its primary purpose the removal of moisture and did not specifically quantitatively test for moisture removed. The prior art process of removing Fluorinert™ is believed to have unwittingly and unknowingly removed much of the moisture absorbed during the manufacturing process as well as dust and debris left in the heat exchanger, leaving a quantity of tolerable moisture as well as a relatively clean heat exchanger. This tolerable moisture included intrinsic moisture that could not be removed without removing volatile organic plasticizers and organic materials. This tolerable moisture and intrinsic moisture did not significantly impair the normal expected 500 hour MTBF rate. The standard practice of heating and evacuation through the Schrader valve at vacuums typically of 2 Torr does not remove the bulk of the moisture absorbed during the immediately preceding repair operation and is believed not to remove embedded moisture or moisture that was absorbed during previous repair operations.
The invidious nature of the absorbed and embedded moisture in the high voltage high frequency power supply was first recognized by the inventors after discovering the surprising amount of water removed from a high voltage high frequency power supply from an FCR APG-68 defective unit from a B-1 bomber as will be described hereinafter in greater detail. The amount of water removed as moisture is believed to have been deeply embedded in the organic components of the high voltage high frequency power supply. On the ground at a constant temperature the moisture content of the vapor space in the pressure vessel approaches an equilibrium with the moisture content of the organic and inorganic solid state materials in the high voltage high frequency power supply.
The time between flights would allow this equilibrium to be approached at sometimes high ground temperatures. However the rapid change in temperature encountered in flight level altitudes which change at about 1.4 degrees Centigrade per 1,000 feet can drop temperatures by 15° C. in about 10 seconds. Such a rapid cooling due to a rapid change in altitude results in a rapid rise in the relative humidity in the sealed high voltage high pressure vessel. As the relative humidity in the pressure vessel rises rapidly and exceeds 100% condensation would occur resulting in arcing, partial discharges and failure of the FCR APG-68 dual mode transmitter.
The deleterious effect of moisture on the electrical components and properties of insulators is well known. Camilli U.S. Pat. No. 2,300,910 refers to the vacuum treatment and drying to remove all moisture prior to the impregnation of the paper insulation in high voltage windings of transformers during their manufacture. Similarly, Camilli U.S. Pat. No. 2,168,154 provides for drying of the core and windings of a transformer in a partial vacuum and Kolator U.S. Pat. No. 3,587,168 provides for the use of heat and vacuum in the manufacture of transformers. Temperatures in the range used for transformers are beyond the range that are tolerated by FCR APG-68 tactical radar units.
Many methods have been proposed for the drying of electronic components during manufacture such as Wennerstrum U.S. Pat. No. 4,882,851 which discloses the use of microwave heating. Microwave heating cannot be applied to an assembled FCR APG-68 tactical radar dual mode transmitter. Other prior art such as Schroder U.S. Pat. No. 5,189,581 discloses use of a desiccant for removing moisture from the housing of a videocassette recorder.
Leech U.S. Pat. No. 5,433,020 discloses use of a cold trap with a valve between vacuum pump and trap to maintain a fixed differential pressure to control flow rate during the vacuum drying of an object. In contrast the system of the invention employs a valve between cold trap and vacuum chamber to permit measurement of the rate of evolution of embedded and absorbed moisture.
Schober U.S. Pat. No. 3,792,528 dries windings of high voltage transformers, seals them, washes out the sealant and dries the transformer with kerosene vapor before filling with transformer oil. Kerosene vapor cannot be employed to dry FCR APG-68 tactical radar transmitters because of the difficulty in complete removal of the kerosene prior to filling with SF6.
Inoue Tamotsu JP 61 174 707 improves the dielectric strength of the gas of a gas-filled transformer by intermittently circulating the gas through an external drier. This is not practical in an air-borne FCR APG-68 dual mode radar transmitter because the length of time required is so much greater than through the use of vacuum.
Ikuyo and Hiroyuki JP 11 329 328 employs a preliminary chamber to remove surface moisture by electron beam processing and applies heat and vacuum. Subsequently the samples are moved to a separate chamber. It is not practical to move units from one chamber to another for separate treatment due to the reacquisition of atmospheric moisture.
Michio, et al. JP 1110 2829 reduces the rate at which paper insulation deteriorates by heating electrical equipment under vacuum by passing current through the windings. This method of heating the windings is not practical for radar components within the high voltage section, which involve many different components other than transformer windings. Similarly Gmeiner Paul (DE 19 501 323) dries transformers and treats the oil by heating with current through the coils.
Boguslaysky US 2003 0183929 thermally conditions components on IV packages before and/or after repairing them in order to prevent moisture from damaging the packages when subsequently subjected to soldering temperatures. The need to maintain dryness of electrical packages that will be exposed to soldering temperatures for purposes of soldering is very different from removing moisture from FCR APG-68 radar transmitter units to increase their operational life. Dias U.S. Pat. No. 4,347,671 dries the interior of metal surfaces such as tubing for high purity gases by passing through a reactive gas; such a procedure would damage the components of a high voltage high frequency power supply.
The premature failures of repaired FCR APG-68 units have resulted in extensive investigations in the prior art. Arcing and partial discharge and failure have been attributed to the contamination of Coolanol™ which is used as a circulating coolant for the FCR APG-68 tactical radar unit as well as to the contamination of the sulfur hexafluoride gas in the high voltage high frequency power supply.
It has been found by the inventors that failed FCR APG-68 tactical radar units contain contaminated Coolanol™ 25R exhibiting increased color, odor and viscosity and decreased resistivity and in extreme cases sludge. This sludge can be deposited on the heat exchanger surfaces or in the traveling wave tube (TWT). As a result the heat transfer coefficient and the flow rate can decrease because of the formation of solid contaminants that raise the temperature of the TWT, which accelerates the decomposition of the Coolanol™ 25R and the eventual malfunction of the FCR APG-68 tactical radar unit.
Those skilled in the art of FCR APG-68 tactical radar units have extensively investigated Coolanol™ 25R as a source of the problems of arcing, the creation of hot spots and the failure of FCR APG-68 tactical radar units. One study involved the replacement of Coolanol™ 25R with polyalphaolefin under the title Coolanol 25R Replacement for Military Aircraft Cooling Systems AF06-083, which contract was awarded to METSS Corporation of Westerville, Ohio and an Article entitled Methodology for Comparison of Hydraulic and Thermal Performance of Alternative Heat Transfer Fluids in Complex Systems, By Ghajar, Tang and Beam, Vol. 16, Issue 1 January-March 1995 Heat Transfer Engineering. 
Those skilled in the art have also investigated the FCR APG-68 tactical radar unit as a function of the purity of sulfur hexafluoride (SF6) or its contamination. SF6 purity is important since the electronics package of the high voltage high frequency unit is sealed in an atmosphere of SF6. There is however disagreement in the literature on the effect of moisture on the behavior of SF6 in arcing and corona discharge.
In the prior art heat transfer surfaces in TWT's and in heat exchangers that are in contact with the liquid heat transfer medium Coolanol™ 25R have been at least partially cleaned by circulation of Coolanol™ through filters during fluid replacement. No means have been employed in the prior art to clean heat transfer surfaces that are in contact with air or residue from Coolanol™ or other fluids used during the original manufacture or repair processes. In APG-68 tactical radar units other than those used in B-1 bombers, heat is transferred from electronic components mounted on a cold plate to air flowing through channels in said cold plate and heat is transferred from Coolanol™ to air in heat exchangers. These heat transfer surfaces in contact with air accumulate films with increasing amounts of dirt and debris and spent jet fuel hydrocarbons and residue with increasing service, resulting in reduced heat transfer, decreasing MTBF and eventual malfunction of the FCR APG-68 tactical radar units. Furthermore dirt, residue and contamination from these heat transfer surfaces has also in the prior art been transferred inadvertently to the surfaces of electronic components during the high voltage corona testing step that follows repair operations.
The deleterious effect of excessive temperature on electronic components is well known, as is the adverse effect of dirt and other contaminants on the transfer of heat through surfaces of heat exchangers. Rosin U.S. Pat. No. 7,789,970 optimizes the cleaning of heat exchange surfaces by measuring the particles in the exhaust stream. Takashima U.S. Pat. No. 7,485,612 discloses an electronic parts cleaning solution that removes fine dust and organic matters adhered to the surface of electronic parts but employs chemicals that would be entirely unacceptable for use on parts of equipment subject to high voltages. Kishimoto U.S. Pat. No. 6,893,530 discloses a method and system for cleaning to remove cutting dust and the like adhering to board material and drying. The method involves repeatedly evacuating and backfilling with air, each evacuation proceeding to a lower pressure than the previous evacuation.
As a result those skilled in the art have considered various options to remedy the premature ageing and high rate of failure of FCR APG-68 tactical radar units. The initial cost of acquisition at almost one million dollars a unit and their reduced service life and requirements for repair and maintenance have provided a great incentive for finding an acceptable method or procedure for remediating and upgrading the performance of these vital tactical radar units.
In the prior art repair process Fire Control Radar FCR APG-68 units are repaired and as in the original manufacture of the transmitters during which the partially assembled electronic assemblies are tested for corona discharge in baths of Fluorinert™. In the original manufacture the units were also rotated in a bath of circulating Fluorinert™ to remove contaminants resulting from the repair process. The circulating Fluorinert™ employed in the original manufacture of the units was sufficient to remove any contamination left over from the original manufacture of the units but would be insufficient to clean units that have been in service. In the prior art no action was taken to clean the air-cooled heat transfer surfaces of repaired transmitters.